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72 lines
3.1 KiB
72 lines
3.1 KiB
% Casts
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Casts are a superset of coercions: every coercion can be explicitly
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invoked via a cast. However some conversions require a cast.
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While coercions are pervasive and largely harmless, these "true casts"
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are rare and potentially dangerous. As such, casts must be explicitly invoked
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using the `as` keyword: `expr as Type`.
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True casts generally revolve around raw pointers and the primitive numeric
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types. Even though they're dangerous, these casts are infallible at runtime.
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If a cast triggers some subtle corner case no indication will be given that
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this occurred. The cast will simply succeed. That said, casts must be valid
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at the type level, or else they will be prevented statically. For instance,
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`7u8 as bool` will not compile.
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That said, casts aren't `unsafe` because they generally can't violate memory
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safety *on their own*. For instance, converting an integer to a raw pointer can
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very easily lead to terrible things. However the act of creating the pointer
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itself is safe, because actually using a raw pointer is already marked as
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`unsafe`.
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Here's an exhaustive list of all the true casts. For brevity, we will use `*`
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to denote either a `*const` or `*mut`, and `integer` to denote any integral
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primitive:
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* `*T as *U` where `T, U: Sized`
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* `*T as *U` TODO: explain unsized situation
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* `*T as integer`
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* `integer as *T`
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* `number as number`
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* `C-like-enum as integer`
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* `bool as integer`
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* `char as integer`
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* `u8 as char`
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* `&[T; n] as *const T`
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* `fn as *T` where `T: Sized`
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* `fn as integer`
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Note that lengths are not adjusted when casting raw slices -
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`*const [u16] as *const [u8]` creates a slice that only includes
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half of the original memory.
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Casting is not transitive, that is, even if `e as U1 as U2` is a valid
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expression, `e as U2` is not necessarily so.
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For numeric casts, there are quite a few cases to consider:
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* casting between two integers of the same size (e.g. i32 -> u32) is a no-op
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* casting from a larger integer to a smaller integer (e.g. u32 -> u8) will
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truncate
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* casting from a smaller integer to a larger integer (e.g. u8 -> u32) will
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* zero-extend if the source is unsigned
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* sign-extend if the source is signed
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* casting from a float to an integer will round the float towards zero
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* **[NOTE: currently this will cause Undefined Behaviour if the rounded
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value cannot be represented by the target integer type][float-int]**.
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This includes Inf and NaN. This is a bug and will be fixed.
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* casting from an integer to float will produce the floating point
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representation of the integer, rounded if necessary (rounding strategy
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unspecified)
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* casting from an f32 to an f64 is perfect and lossless
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* casting from an f64 to an f32 will produce the closest possible value
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(rounding strategy unspecified)
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* **[NOTE: currently this will cause Undefined Behaviour if the value
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is finite but larger or smaller than the largest or smallest finite
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value representable by f32][float-float]**. This is a bug and will
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be fixed.
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[float-int]: https://github.com/rust-lang/rust/issues/10184
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[float-float]: https://github.com/rust-lang/rust/issues/15536
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